Optical absorption edge characteristics of cubic boron nitride thin films

Boron nitride films with different cubic phase contents were deposited in a radio frequency bias sputtering system by a two-stage deposition process. The Fourier transform infrared spectra and UV-visible transmittance and reflection spectra were measured. The optical absorption edge of BN films consists of a low energy region in which α increases exponentially as incident photon energy $hν,$ and a high energy region, in which α varies as $(hν)m,$ which is the characteristics of amorphous materials. These two regions are fitted by the Urbach tail model and the band-to-band transition model with an effective medium approach, and the Urbach energy and the optical band gap are determined from these fits. With an increase in the cubic boron nitride $(c-BN)$ content, the absorption edge shifts to the higher energy and the optical band gap increases. For a BN film with 88% cubic phase, the optical band gap exceeds 6.0 eV, which is comparable to that of $c-BN$ single crystal.